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Th e evolution of increased competitive ability (EICA) hypothesis provides a compelling explanation for the success of invasive species. It contends that because alien plants have escaped their coevolved natural enemies, selection pressures favor a diversion of resources from herbivore defense to traits that confer increased competitive ability. Here, we provide evidence for EICA in the noxious grassland invader Lespedeza cuneata , by comparing the ancestral genotype introduced to North America in 1930 with modern-day invasive (North American) and native (Japanese) genotypes. We found that the invasive genotype was a better competitor than either the native or the ancestral genotype. Further, the invasive genotype exhibited greater induced resistance but lower constitutive resistance than the ancestral and native genotypes. Our results suggest that selection has played a pivotal role in shaping this invasive plant species into a more aggressive, but less constitutively defended competitor. Th e rapid population growth and high densities reached by invasive species provides scientists with many opportunities to investigate the mechanisms that regulate the abundance and distribution of organisms. Such research is a conservation priority since invasive species cause signifi cant environmental and economic damage, and pinpointing the mechanisms that allow invasive species to achieve community dominance

The effects of early-season herbivory and subsequent induced plant responses have the potential to affect the diversity of herbivorous insect communities. We investigated the seasonal development of the herbivore fauna on common milkweed (Asclepias syriaca) to understand the effect of early-season herbivory by different species on insect growth, natural colonization, and community composition. First, we showed that damage by an early-season stem-feeding weevil (Rhyssomatus lineaticollis) reduced growth of monarch larvae (Danaus plexippus) and leaf beetle larvae (Labidomera clivicollis), suggesting that plant quality is reduced by weevil damage. To better understand the potential for initial herbivore damage to affect subsequent colonization by herbivores in the field, we compared undamaged controls to plants experimentally damaged with one of three herbivores: weevils, monarchs, or leaf beetles. We counted seven species of naturally colonizing herbivores on all plants for the next two months to assess colonization, damage, and insect community richness. Our results showed that initial herbivory by different species altered host plant use by herbivores in two years of experiments. Similarly, induced resistance and suscep- tibility occurred in both years, but due to different initial damaging species on individual plants. Treatment effects also scaled up to alter herbivore community richness. Initial treatments varied in their persistence through the season. For example, in 2001, the influence of initial monarch damage dissipated due to subsequent damage by colonizing herbivores, but the impacts of initial weevil treatment were unaffected. This result suggests that, al- though induced responses to weevil feeding persisted through the season, monarch herbivory was more likely to affect the herbivore community via a cascade of indirect effects. In 2002, plant and insect responses were more specific, depending on the identity of both initial and colonizing herbivore species. Despite year-to-year variation, considerable con- sistency in many responses to our treatments indicates that the identity of the initially colonizing herbivore can affect subsequent plant use and community structure. Given the preponderance of influential early-season herbivores, the effects of induced plant responses similar to those presented here may be widespread and may strongly contribute to the structure of phytophagous insect communities.

Induced plant responses to herbivory appear to be universal, yet the degree to which they are specific to sets of herbivores is poorly understood. The generalist/specialist hypothesis predicts that generalist herbivores are more often negatively affected by host plant defenses, wheras specialists may be either unaffected by or attracted to these same “plant defenses”. Therefore, specialists should be less predictable than generalists in their responses to induced plant resistance traits. To better understand the variation in plant responses to herbivore attack, and the impacts these responses have on specialist herbivores, we conducted a series of experiments examining pairwise interactinos between two specialaist herbivores of the common milkweed (Asclepias syriaca). We damaged plants mechnically, with swamp milkweed beetles (Labidomera clivicollis), or with monarchs (Danaus plexippus), and then asessed specificity of elicitation, both by measuring a putative defensive trait (latex volume) and by challenging plants with insects of both species in bioasays. Latex production increased by 34% and 13% following beetle and monarch herbivory, respectively, but only beetles significantly elevated latex production compared to undamaged controls. While beetle growth was negatively affected by latex across all experiments, beetles were not affected by previous damage caused by conspecifies or by monarchs. In contrast, monarchs feeding on previously damaged plants were 20% smaller, and their response was the same on plants damaged mechnically or by either herbivore. Therefore, these specialist herbivores exhibit both specificity of elicitation in plant responses and specificity of effects in response to prior damage.

Summary 1 We studied the genetic and environmental determinants of attack by the specialist stem-attacking weevil, Rhyssomatus lineaticollis on Asclepias syriaca. 2 In natural populations, the extent of stem damage and oviposition were positively correlated with stem width, but not stem height. We hypothesized that both genotypic and environmental factors influencing stem morphology would affect attack by weevils. 3 In a common garden study with 21 full-sib families of milkweed, both phenotypic and genetic correlations indicated that weevils impose more damage and lay more eggs on thicker stemmed plants. 4 Of three other putative resistance traits, only latex production showed a negative genetic correlation with weevil attack. 5 When neighbouring grasses were clipped to reduce light competition, focal milkweed plants received up to 2.6 times the photosynthetically active radiation and 1.6 times the red to far red ratio of light compared with plants with intact grass neighbours. Focal milkweed plants were therefore released from the classic neighbour avoidance response and had 20% shorter internode lengths, were 30% shorter, and had 90% thicker stems compared with controls. 6 Clipping of grass neighbours resulted in nearly 2.7 times the damage and oviposition by stem weevils, thus supporting the hypothesis of an environmental or trait-mediated indirect influence on resistance. 7 Although attack of plants by weevils strongly increases the probability of stem mortality, thicker stems experience lower mortality, thus counteracting the selective impact of weevil-induced plant mortality. 8 The determinants of attack on milkweeds include both genetic variation for stem thickness and an indirect environmental influence of plant neighbours. If milkweeds and weevils are coevolving, the interaction is diffuse because the ecological neighbourhood is likely to modify the patterns of reciprocal natural selection.

Summary 1 Salinization is a growing environmental stress in wetland ecosystems world-wide. Several models have been proposed that predict clonal plant responses to stress, including that environmental stress stimulates sexual reproduction. 2 We conducted a common-garden experiment to investigate the effects of salinity on 10 natural populations of Iris hexagona, a clonal perennial endemic to freshwater and brackish wetlands of the North American Gulf Coast. 3 Salinity reduced vegetative growth but either increased or had neutral effects on sexual reproduction, consistent with the clonal stress hypothesis. Salinity of 4 µg g−1 more than doubled the number of seeds produced compared with freshwater controls, but flower number and seed mass were unaffected. 4 Salinity reduced total below-ground mass by nearly 50% compared with controls, with no significant change in rhizome numbers. 5 Plants from 10 randomly selected I. hexagona populations differed dramatically in growth and reproduction, independent of salinity. Total biomass that accumulated over the 20-month experiment ranged across all treatments from 52 to 892 g, and flower numbers varied from 2.3 to 11.3 per replicate. 6 Populations did not respond differently to salinity, except with respect to above- : below-ground ratios, thus providing no conclusive evidence for local adaptation to salinity stress. 7 Our results concur with published models of plant reproductive strategies in variable environments, in that environmental stress stimulated sexual reproduction at the expense of growth. However, these models do not predict the observed sharp decline in seed production at near lethal salinity levels.

Stilbosis quadricustatella leafminers are microlepidopteran specialists of sand-live oak (Quercus geminata). These tiny moths produce one generation per year and have a parasitic life-cycle and long larval stage that develops entirely within a single oak leaf. Differences in host-plant age, phenotype, and phenology generate a coarse-grained, spatially heterogeneous environment for the leafminer population. Previous reciprocal trans- fers of leafminer eggs among mature oaks revealed that S. quadricustatella are locally adapted to individual oak trees. In this paper we use genetic markers and an extinction- recolonization experiment to explore further variation in leafminer population structure. Allozyme loci indicate significant interdemic genetic structure among recent colonists of new host trees, which weakens in the 10th generation and disappears by the 40th generation. In contrast, adaptive demic structure is evident by the 10th generation and is strong in the 40th generation, despite the potential for substantial intertree dispersal. We propose that host heterogeneity combined with leafminer fidelity to natal trees promotes divergent se- lection and rapid demic evolution on individual oaks, despite potentially high gene flow between the leafminers inhabiting them.